In a normal lung pulmonary ventilation (VA) and perfusion (Q) are closely matched to optimize the gas exchange efficiency. Various disease states can alter this normal relationship and cause a heterogeneous VA/Q ratio mismatch. The assessment and quantification of regional VA/Q and regional oxygen partial pressure are of paramount importance to the diagnosis, evaluation, and treatment of a variety of pulmonary disorders. Recently, hyperpolarized gas MRI methods have been developed to study ventilation and/or perfusion abnormalities. Regional maps of functional lung parameters were acquired and the changes in these parameters caused by physiologic defects were investigated. The newly developed hyperpolarized gas MRI techniques overcome many of the difficulties associated with traditional radiological diagnostic tools and promises the comprehensive and accurate determination of regional lung function. Recently, VA/Q data obtained by this MRI technique has been compared to nuclear medicine. However, ultimate clinical use of this method demands a careful validation against a gold standard on a smaller scale. Therefore, the main goal of this proposal is to validate the hyperpolarized gas MRI technique for the generation of maps of the regional ventilation/perfusion ratios and maps of the alveolar partial pressure of oxygen on a local scale. We hypothesize that these maps accurately reflect the values obtained from simultaneous measurements of ventilation and perfusion with inhaled and injected microspheres. To achieve the stated goal, maps of VA/Q and PAO2 will be acquired using the MRI technique as well as the microsphere technique in a healthy rabbit model. Key Words: Polarized Gas MRI; Regional Ventilation; Regional Perfusion; Pegional PAO2 [unreadable] [unreadable] [unreadable]